Optimization of sintering conditions in bulk MgB₂ material for improvement of critical current density

The present investigation focuses on methods to further improve the J_c values of disk-shaped bulk MgB₂ superconductors by optimizing the sintering conditions. We prepared two sets of bulk MgB₂ material from commercial high-purity powders of Mg metal and amorphous B using a single-step solid-state reaction process. To optimize the sintering time, a set of samples was sintered at 775 °C with sintering duration ranging between 1 and 10 h (pure Ar atmosphere). A second set of samples was produced similarly at 775, 780, 785, 795, 800 and 805 °C (3 h, pure argon atmosphere). X-ray diffraction analysis showed that both sets of samples were single phase MgB₂. Magnetization measurements confirmed a sharp superconducting transition with T_c,onset ≈ 38.2 K-38.8 K. The critical current density (J_c) values for MgB₂ samples produced for 1 h were the highest in all processed materials, i.e., the high J_c value of 270,000 A/cm² and 125,000 A/cm² (20 K, self-field and 1 T) were achieved in the sample produced at 775 °C, without any additional doping. In contrast, the second series of samples clearly indicated that at 805 °C (3 h) the highest J_c of 245,000 A/cm² and 110,000 A/cm² (20 K, self-field and 1 T) were achieved. AFM and EBSD observations indicated that largest amount of fine grains do exist in the sample sintered at 775 °C, but the narrowest distribution of grains does exist in the sample sintered at 800 °C. The present results clearly demonstrate a strong relation between the microstructure and the pinning performance. The optimization of the sintering conditions is crucial to improve the performance of bulk MgB₂ samples.